Taking an excavator as an example, the prior art associated therewith will be described below.
The excavator generally employs a hydraulic motor driving system that is designed to use a hydraulic motor as a rotation driving source and to drive this hydraulic motor by discharge oil from a hydraulic pump.
In the case of employing this system, the discharge oil has its direction, pressure, and flow rate controlled by a control valve disposed between the hydraulic pump and the hydraulic motor, so that the operating direction, force, and speed of the hydraulic motor are controlled.
This system, however, has a drawback that a large amount of hydraulic energy has to be squeezed and disposed of by the control valve, resulting in huge loss of energy.
Thus, an electric motor driving system has been proposed that employs an electric motor as the rotation driving source (see Japanese Unexamined Patent Publication No. 11-93210, for example).
Conventionally, a large electric excavator for a climbing crane or for mining also employs the electric motor driving system in a rotation operation.
In such an electric motor rotation driving system, the direction and speed of rotation of a rotating body is controlled by adjusting the rotation direction and rotating speed of the electric motor, thereby significantly improving energy efficiency.
At the same time, when employing this system, a feedback speed control system is normally used to control the speed of the rotating body such that a deviation between an actually measured speed of the rotating body and a target speed thereof corresponding to an operation amount of an operation means is eliminated.
This system, however, has the following drawback concerning operational performance in rotation.
When the operation means is located in a neutral position and a commanded speed equals zero, braking torque is produced to stop a rotating body. Once a rotating speed of the electric motor becomes zero, torque (stopping and holding force) that causes the zero speed to be maintained will never be outputted, which results in the fact that braking and holding effect cannot be surely obtained.
For this reason, as a control system for stopping and holding the body is proposed another system that employs a mechanical brake mounted on a working machine with a hydraulic driving system.
The mechanical brake, however, is basically designed to actuate as a parking brake when the rotating body is in a stopped state. If this brake is used as a means for decelerating and stopping the electric motor in the electric motor rotation driving system without modification, there arises a problem that brake wearing becomes severe, while the jerky movement of the rotating body is caused due to shocks from an on/off operation of the brake when decelerating and accelerating, so that a smooth rotation stopping/accelerating effect cannot be obtained, thus degrading its operability.
On the other hand, in a normal operation of rotating an upper rotating body 2 above ground, a feedback speed control system enables control of rotating speed according to the operation amount of the operation means, which has no operational problem.
In contrast, when performing a pressing work which involves excavating earth by a bucket 6 and pressing its side against a wall surface g1 in a groove g to form the pressed wall surface g1, as shown in FIG. 14, the speed of rotation around a rotating shaft O becomes appropriate zero. Under the feedback speed control, a deviation between a target value of the rotating speed and an actually measured value thereof is increased. As a result, the feedback effect causes maximum rotation torque (electric motor torque) even when a slight amount of bucket operation is taken.
For this reason, when performing such a pressing work through the rotation, the torque control carried out by an operator becomes impossible, thereby impairing the machine operability.
Therefore, it is desirable that the feedback speed control system is employed, while imposing a limitation on torque depending on the operation amount.
When imposing the limitation on the torque as described above, the small operation amount of the operation means results in small torque of the electric motor. Therefore, when the rotating body starts to rotate toward an upper end side of an inclined ground or when the rotating body starts to rotate upwind under strong wind, the torque limitation as described above produces a shortage of accelerating torque. This causes the rotating body to rotate in a reverse direction, i.e. so-called “retrograde motion”, disadvantageously leading to degradation in its security and operability.
Accordingly, the present invention is to solve the foregoing problems, and it is an object of the present invention to provide a rotation control device for a working machine with improved operability in rotation.
Concretely, it is a first object of the invention to securely hold the rotating body in a stopped state, to smoothly carry out a decelerating and stopping function of the rotation and an accelerating effect thereof without energy loss for stopping and holding the body, and to make it possible to use a known mechanical brake as it is.
It is a second object of the invention to prevent retrograde motion of the rotating body, which might be caused due to the shortage of torque, while imposing a limitation on the torque.
It is a third object of the invention to enable rotating torque control in the pressing work.